The delivery of dermatological products to target cells within the layers of skin has been hampered by the variable rates of absorption and metabolism obtained by oral treatments and by absorption of the product into the blood stream or failure to be absorbed at all in transdermal drug delivery treatments. A large majority of dermatologic conditions have been traditionally treated by topical administration of drug and skin treatment agents. However, the skin's surface provides a natural barrier to these agents, often making effective topical delivery difficult. Topical administration is nevertheless often preferred to oral administration of drugs and other agents to treat dermatologic problems because substantially less drug enters the blood stream and systemic side effects can be eliminated or significantly reduced.
Various topical drug delivery systems are known in the art. These topical delivery systems generally fall into one of several major categories. A first category consists of traditional formulating agents such as, for example, ethanol, propylene glycol, various fatty compounds, and surfactants. Under certain conditions, these agents can function as penetration enhancers, but they generally do not provide prolonged duration of action of drugs or skin care agents. Frequently, they are not very effective unless used in doses that may cause skin irritation or other adverse side effects, or conversely may produce excessive and poorly controlled absorption, in particularly into the vascular system. A second category of skin care delivery systems are those that utilize specialized delivery technologies. These include, for example, liposomes, microspheres, transdermal patches and iontophoresis. Liposomes (a liquid encapsulation system) and microspheres (a solid entrapment system) require encapsulation of the drug or skin care agent, which may be complex, costly, or may introduce drug stability problems. Transdermal patches and iontophoresis can be used to treat dermatologic disorders, but these technologies may be impractical to use at multiple skin sites or on certain areas, such as the face. A third major category of known topical drug delivery systems includes non-invasive long-lasting liquid reservoirs for holding and depositing therapeutic or cosmetic agents in and on the stratum corneum and epidermis. An example of this type of topical delivery system are polyolprepolymers, which are hydroxy-terminated polyalkylene glycol-based polyurethane polymers that are capable of forming reservoirs in the upper layers of the skin, to hold and deposit therapeutic or cosmetic agents in an on the stratum corneum and epidermis. U.S. Pat. No. 4,971,800, 5,045,317 and 5,051,260, all to Chess, et al., relate to these compositions and methods. However, these polymeric agents tend to be viscous materials which are difficult to spread on the skin, and exhibit considerable drag, oily feel and tackiness.
In selecting a topical drug delivery system it is essential that adverse systemic effects and irreversible damage to the skin structure be avoided. It is also desirable that the compound itself not cause irritation or allergic response and that the delivery system is resistant to accidental removal and remains on and in the upper layers of the skin for an extended period of time. The delivery systems should also provide acceptable feel and spreadability. It is also desirable that the polymeric drug delivery system be compatible with a wide range of "active" agents and formulation ingredients.